The determination of the presence and/or quantity of oxygen in the gas phase is essential to the efficient operation of many processes and devices, particularly those involving combustion. Monitors currently in use provide analyses based on three principles: i) spectroscopy of the gas phase, ii) amperometric properties of certain types of electrolytic cells and iii) surface chemistry of gas adsorption onto selected solid materials (ie. Chemical Sensing). The first two involve expensive and bulky apparatus and can, with careful calibration, produce accurate analyses. The latter technique potentially provides cheap, small and robust devices, but a main deficiency of the analysis is non-specificity to the analyte gas, with consequent interferences with other gases present in the analyte gas stream.
U.S. Pat. No. 4,314,996 granted Feb. 9, 1982 to Sekido et al describes an oxygen sensor element comprising a substrate made of a compound oxide of the perovskite type and a pair of electrodes electrically connected to the substrate. The compound oxide has a formula ABO.sub.3 in which A represents an element of the lanthanum family, an alkaline earth metal or a mixture thereof and B is a transition metal. The layer of the oxide is about 0.4 mm thick.
U.S. Pat. No. 4,608,232 issued Aug. 26, 1986 to Sunano et al describes a gas sensor which has an electrically insulating base, electrodes formed on the base, and a gas sensitive film formed on the surface of the base at a position where it detects oxygen gas. The gas sensitive film is a thermally sprayed film which is made of an oxide having a perovskite structure. The surface of the film is formed with fine cracks which increase the effective surface area and enhance the Speed of response of the sensor.
U.S. Pat. No. 5,071,626 issued Dec. 10, 1991 to Tuller describes an oxygen sensor having a sensing element of a copper oxide ceramic with perovskite related structure. The conductivity of the sensing element is a function of oxygen partial pressure. A preferred ceramic is the Y-Ba-Cu-O system. A suitable thickness of the sensor material is indicated to be 0.1-250 .mu.m. The technique used to apply the film is spin-coating and pyrolysis. Alternatively, sputtering and vapor deposition is suggested, but no data or evidence for the sensing quality of the product which would be obtained by this route is provided in the disclosure.
There are many examples of metal oxides and doped analogues being used as sensor materials for the detection or analysis of a variety of volatile compounds. The mode of action of these devices depends upon surface chemistry of the sensing material, and the mechanism, which is of redox character, is not thoroughly understood. It is noted that some of the prior art sensors (see U.S. Pat. No. 4,608,232) react similarly to oxygen and to moisture (ie. cross interference). The reaction of the compound oxide of U.S. Pat. No. 4,314,996 is indicated to be EQU LaCoO.sub.3 .revreaction.La.sub.2 O.sub.3 +Co
The above-mentioned Sunano, Sekido and Tuller inventions use an approach described as thick film technology to produce the sensor surface, whereby a slurry of powdered perovskite and additives are sintered at high temperature onto a supporting substrate as a material which has ceramic physical properties.